For nanotechnological applications, the use of thin films of materials is highly preferred. See, e.g., Segalman, R. A. Mater. Sci. Eng. 2005, R48, 191; Li, M.; Ober, C. K. Materials Today 2006, 9, 30; Hawker, C. J.; Russell, T. P. MRS Bulletin 2005, 30, 952; and Li, M.; Coenjarts, C. A.; Ober, C. K. Adv. Polym. Sci. 2005, 190, 183. It is desirable, in the case of block copolymers (BCPs), to have the nanoscopic domains, sometimes referred to as microdomains, oriented in a specific direction with a long-range lateral order for applications such as polarizers (Pelletier, V.; Asakawa, K.; Wu, M.; Adamson, D. H.; Register, R. A.; Chaikin, P. M. Appl. Phys. Lett. 2006, 88, 211114), templates for pattern transfer to generate microelectronic integrated circuits (Black, C. T. IEEE Trans. Nanotechnol. 2004, 3, 412), magnetic media (Ross, C. A. Annu. Rev. Mater. Res. 2001, 31, 203), and optical waveguides (Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. Adv. Mater. 2001, 13, 1501; Kim, D. H.; Lau, K. H. A.; Robertson, J. W. F.; Lee, O.-J.; Jeong, U.; Lee, J. I.; Hawker, C. J.; Russell, T. P.; Kim, J. K.; Knoll, W. Adv. Mater. 2005, 17, 2442).
In recent years, a number of approaches have been developed to control the orientation and enhance the lateral order of the microdomains by applying external fields, such as electric fields (Thorn-Albrecht, T.; DeRouchey, J.; Russell, T. P. Macromolecules 2000, 33, 3250), shear (Villar, M. A.; Rueda, D. R.; Ania, F.; Thomas, E. L. Polymer 2002, 43, 5139), temperature gradients (Bodycomb, J.; Funaki, Y.; Kimishima, K.; Hashimoto, T. Macromolecules 1999, 32, 2075), graphoepitaxy (Segalman, R. A.; Yokoyama, H.; Kramer, E. J. Adv. Mater. 2001, 13, 1152), chemically patterned substrates (Stoykovich, M. P.; Müller, M.; Kim, S. O.; Solak, H. H.; Edwards, E. W.; de Pablo, J. J.; Nealey, P. F. Science 2005, 308, 1442), controlled interfacial interactions (Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. J. Science 1997, 275, 1458; Drockenmuller, E.; Li, L. Y. T.; Ryu, D. Y.; Harth, E.; Russell, T. P.; Kim, H. C.; Hawker, C. J. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 1028), zone casting (Tang, C.; Tracz, A.; Kruk, M.; Zhang, R.; Smilgies, D.-M.; Matyjaszewski, K.; Kowalewski, T. J. Am. Chem. Soc. 2005, 127, 6918), optical alignment (Morikawa, Y.; Nagano, S.; Watanabe, K.; Kamata, K.; Iyoda, T.; Seki, T. Adv. Mater. 2006, 18, 883), solvent fields (Kim, G.; Libera, M. Macromolecules 1998, 31, 2569; Kimura, M.; Misner, M. J.; Xu, T.; Kim, S. H.; Russell, T. P. Langmuir 2003, 19, 9910; Ludwigs, S.; Baer, A.; Voronov, A.; Rehse, N.; Magerle, R.; Krausch, G. Nature Mater. 2003, 2, 744; Kim, S. H.; Misner, M. J.; Xu, T.; Kimura, M.; Russell, T. P. Adv. Mater. 2004, 16, 226; Ho, R.-M.; Tseng, W.-H.; Fan, H.-W.; Chiang, Y.-W.; Lin, C.-C.; Ko, B.-T.; Huang, B.-H. Polymer 2005, 46, 9362; Lin, Z.; Kim, D. H.; Wu, X.; Boosanda, L.; Stone, D.; LaRose, L.; Russell, T. P. Adv. Mater. 2002, 14, 1373; Hahm, J.; Sibener, S. J. Langmuir 2000, 16, 4766; and Park, S.; Kim, B.; Wang, J.-Y.; Russell, T. P. Adv. Mater. 2008, 20, 681), and so on.
Solvent evaporation is a strong and highly directional field. Making BCP thin films under various solvent evaporation conditions has been found to be a good way to manipulate the orientation and lateral ordering of BCP microdomains in thin films Kim et al. first reported that solvent evaporation could be used to induce the ordering and orientation of BCP microdomains. Kim, G.; Libera, M. Macromolecules 1998, 31, 2569. Vertically aligned cylindrical PS microdomains could be obtained in polystyrene-b-polybutadiene-b-polystyrene triblock copolymer thin films with a thickness of ˜100 nm The same effect was also observed with polystyrene-b-poly(ethylene oxide) (PS-b-PEO) and polystyrene-b-poly(L-lactide) BCP thin films and was attributed to a copolymer/solvent concentration gradient along the direction normal to the film surface giving rise to an ordering front that propagated into the film during solvent evaporation. Ho, R.-M.; Tseng, W.-H.; Fan, H.-W.; Chiang, Y.-W.; Lin, C.-C.; Ko, B.-T.; Huang, B.-H. Polymer 2005, 46, 9362; Lin, Z.; Kim, D. H.; Wu, X.; Boosanda, L.; Stone, D.; LaRose, L.; Russell, T. P. Adv. Mater. 2002, 14, 1373. This orientation was independent of the substrate. However, the lateral ordering of the cylindrical microdomains was poor. Hahm et al. (Hahm, J.; Sibener, S. J. Langmuir 2000, 16, 4766) and later Kimura et al. (Kimura, M.; Misner, M. J.; Xu, T.; Kim, S. H.; Russell, T. P. Langmuir 2003, 19, 9910) showed that evaporation-induced flow, in solvent-cast BCP films, produced arrays of nanoscopic cylinders oriented normal to the surface with a high degree of ordering. Recently, Ludwigs et al. demonstrated that solvent annealing could markedly enhance the ordering of BCP microdomains in thin films Ludwigs, S.; Böker, A.; Voronov, A.; Rehse, N.; Magerle, R.; Krausch, G. Nature Mater. 2003, 2, 744. By controlling the rate of solvent evaporation and solvent annealing in thin films of PS-b-PEO, Kim et al. achieved nearly-defect-free arrays of cylindrical microdomains oriented normal to the film surface that spanned the entire film Kim, S. H.; Misner, M. J.; Xu, T.; Kimura, M.; Russell, T. P. Adv. Mater. 2004, 16, 226. Moreover, the use of a co-solvent enabled further control over the length scale of lateral ordering. The most recent results showed that perpendicular cylindrical microdomains oriented normal to the film surface could be obtained directly by spin-coating polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) BCPs from mixed solvents of toluene and tetrahydrofuran (THF) and arrays of highly ordered cylindrical microdomains formed over large areas after exposing the films to the vapor of a toluene/THF mixture. Park, S.; Wang, J.-Y.; Kim, B.; Chen, W.; Russell, T. P. Macromolecules 2007, 40, 9059. This process was independent of substrate, but it strongly depends on the quality of the solvents for each block and the solvent evaporation rate. Furthermore, the ultimate achievable data storage density achievable with these BCPs will not exceed 1 terabit/inch2 (0.155 terabit/centimeter2). Processes to use alternate BCPs, like PS-b-P4VP or PS-b-PEO containing salt, have been developed that are simpler to employ and, more importantly, the interactions between the segments of the copolymer are very non-favorable, making defects energetically costly and, also, opening an avenue to smaller domain sizes and separation distances.
Several methods have been developed to prepare nearly perfect patterns onto substrates. Conventional photolithography, electron beam (e-beam) lithography, and scanning force probe lithography are accessible techniques for fabrication of nanometer-size patterns. For example, Schmidt and co-workers showed the successful electrochemical modification of self-assembled monolayers at positions where a conductive scanning probe was in contact with a self-assembled monolayer. Wyrwa, D.; Beyer, N.; Schmid, G. Nano Lett. 2002, 2, 419. The induced chemical contrast was used to guide the covalent binding of Au crystals from solution. E-beam lithography is a common method for fabrication of sub-micrometer structures. Although a beam of electrons may be focused to less than one nanometer in diameter, the resolution is limited by the interaction of the beam with the resist material and by the radius of gyration of the macromolecules, which is usually a few nanometers. Gibson, J. M. Phys. Today 1997, 50, 56. New developments in using self-assembled monolayers overcome these restrictions inherent with standard resist materials as their thickness is usually a few angstroms. Structures as small as a few nanometers have been fabricated by using this concept. See, e.g., Gölzhäuser, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Adv. Mater. 2001, 13, 806; Glass, R.; Arnold, M.; Cavalcanti-Adam, E. A.; Blümmer, J.; Haferkemper, C.; Dodd, C.; Spatz, J. P. New J. Phys. 2004, 6, 101; Yang, S.-M.; Jong, S. G.; and Choi, D.-G. U.S. Pat. No. 7,081,269 (2006). However, these approaches require expensive equipment and high-energy doses and are not suitable for non-conductive substrates, unless additional treatment is carried out. Moreover, e-beam patterning is a time-consuming serial process not suitable for large areas.
It is highly desirable to develop parallel processes where the sequential generation of nanoscopic features is avoided and the patterning is achieved in one-step. Nanoimprint lithography (NIL) is one such process to control the positional order of the microphase separated morphology. NIL can be used for locally controlling the self-assembly process of block copolymers and determining the precise positioning of the phase-separated domains via the topography of mold, rather than the substrate. Li, H.-W.; Huck, W. T. S, Nano Letters 2004, 4, 1633. NIL creates features by a mechanical deformation of a polymer film by pressing a hard mold into the film at temperatures higher than the glass transition temperature of the polymer. This high-throughput, low cost process is not diffraction limited, and sub-10 nanometer resolution has been reported. See, e.g., Chou, S. Y.; Krauss, P. R.; Zhang, W.; Guo, L.; Zhuang, L. J. Vac. Sci. Technol. B 1997, 15, 2897; Li, H.-W.; Huck, W. T. S, Nano Lett. 2004, 4, 1633; Chou, S. Y. U.S. Pat. No. 5,772,905 (1998); and Jeong, J.-H.; Sohn, H.; Sim, Y.-S.; Shin, Y.-J.; Lee, E.-S.; and Whang, K.-H. U.S. Pat. No. 6,943,117 (2005). Yet, NIL has the limitation that it requires a master that is used for the printing and, as of yet, it has not been possible to generate a perfect master with uniform, nanoscopic features sizes less than 20 nanometers over large lateral distances while maintaining the features in register. There remains a need for a parallel process for generating large area, high-density, highly-ordered arrays with feature sizes less than 20 nanometers.